1. Field of the Invention
This invention relates to a position detection device in which a position of an object is detected by changes in the phase of light caused by movement of a diffraction grating.
2. Description of Related Art
As an optical displacement measurement device for detecting the position of relative movement of a mobile part of, for example, a machine tool or a semiconductor manufacturing device, there is known such device configuration exploiting a grating interferometer. This device configuration detects the displacement of a diffraction grating position, as recorded on a movable scale, by taking advantage of light interference.
The present inventors already proposed in, for example, Patent Publication 1, an optical displacement measurement device which, by superimposing two diffracted light beams, which are to interfere with each other, is capable of detecting the position of movement of a movable part to high resolution and high accuracy.
In the technique disclosed in this Patent Publication 1, a coherent light beam forms an optical image on a grating surface of a diffraction grating by a first image-forming means. A first diffracted light beam is collimated by a second image-forming means and illuminated on a reflector of a reflection optical system in a perpendicular direction at all times. With such configuration, the first diffracted light beam as reflected retraces the optical path it followed as the incident light, with there being no change in the optical image forming position on the grating surface of the diffraction grating. On the other hand, the optical axis of a second diffracted light beam, generated by diffraction of the first diffracted light beam, is not offset, while there is no change in the optical path length. In this manner, there is no deterioration in the interference signal for detection even in case the diffraction grating has moved in a direction other than the direction parallel to the grating vector, for example, in case the diffraction grating has become tilted or is suffering from distortions.
In an optical scale for a grating interferometer that detects changes in the light phase caused by movement of the diffraction grating, in order to effect position detection, fingerprints or impurities tend to be affixed to the scale, or the scale may be grazed, if the diffraction grating is exposed to air. Hence, in Patent Publication 2, for example, a transparent protective layer is formed on the surface of the diffraction grating to cover it to protect the surface of the diffraction grating from contamination or grazing.